docs: def _get_basin_depth_term(self, C, sites, period):
          """
          In the case of the base model the basin depth term is switched off.
          Therefore we return an array of zeros.
          """
          if period < 0.65:
              f_dz1 = np.zeros(len(sites.vs30), dtype=float)
          else:
              f_dz1 = C["f7"] + np.zeros(len(sites.vs30), dtype=float)
              f_ratio = C["f7"] / C["f6"]
              dz1 = (sites.z1pt0 / 1000.0) - california_basin_model(sites.vs30)
              idx = dz1 <= f_ratio
              f_dz1[idx] = C["f6"] * dz1[idx]
          return f_dz1